Huge shoals of plankton move from the deep sea and back every day as the sun rises and sets. There are massive migrations of small fish and squid that follow them to exploit this resource, as well as larger predators which hunt them. This enormous movement of biomass from the deep sea to surface and back happens every single day.

Despite the colossal size of this environment, Attenborough very rightly points out that it is still by no means hugely separated from human life. As well as the famous Pacific Garbage Patch that Elin talked about in another post, there is plastic and other marine waste in the most pristine and remote coral reefs. I have heard stories from fellow divers in the Indo-west Pacific about seeing used nappies floating past on dives. I was lucky enough to be involved with a school trip to Baubau near Sulawesi in Indonesia, and we spent a few hours on an uninhabited island cleaning up trash. On another island in Malaysia I found a DVD player and a washing machine on the beach. These are unusual exceptions – polystyrene, plastic bags and straws are ubiquitous in the ocean anywhere in the world. It’s no different in the UK – the Marine Conservation Society at Southampton spend hundreds of hours removing rubbish from beaches on the South coast. When we see pollution in an area we can all agree it is unpleasant, but as a scientist we understand it in context of this colossal, global and unprecedented problem.

This affects all levels of the marine food web. We tend to think of the deep sea as being this remote alien world, but it is still inextricably linked to human life. Microplastics accumulate in deep sea sediments – at 10,000 times higher concentrations than at the surface. Up to 90% of seabirds have plastic in their guts. Another aspect not explored in the programme is that other pollutants dissolved in water – fouling paint, oil and other contaminants – accumulate on plastics, and so make plastic even more toxic to marine life. Pollution becomes more concentrated in higher levels of the food chain in a process known as ‘biomagnification’, where smaller fish with some pollution in them are eaten in large quantities by larger fish. This means that top predators like tuna, sharks and marine mammals are the most contaminated. And as well as being concerning for environmental reasons, the seafood we eat are no exception – plastic has been found in a third of UK-caught fish, and shellfish lovers may consume up to 11,000 plastic particles per year.

Biodegradable plastic is not biodegradable in the sense one might think. These plastics are held together with degradable fibres, so they break down into smaller components. Eventually, they break down into ‘microplastics’, which then spread into every corner of the ocean. It has been suggested that a layer of plastic will be what will distinguish the human era in the fossil record of the future.

It is extremely heartening to see the reactions to this problem, and some countries (most recently Kenya) have even completely banned plastic bags outright. Hopefully Blue Planet will encourage more people than ever to think twice about whether they need that straw or bag, and eventually encourage governments and large companies to move away from the excessive use of this material.

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But what is really astounding about that huge body is that it grows from a single fertilised egg cell, just as our bodies do, but in much the same time as our bodies do. As I’ll try to explain here, that almost makes blue whales impossible animals. But thanks to the recent sequencing of another whale’s genome, perhaps there is a speculative yet intruiging connection to ponder between large body size in whales and breath-hold diving ability.

To attain its huge size, a blue whale’s body grows very rapidly compared with our bodies. A blue whale calf may have a body mass of around 3 tonnes at birth – and yet the time it takes to develop from a single fertilised egg cell (perhaps less than a year for a blue whale) is not very different to our gestation period of 9 months. And after birth, blue whales probably approach their adult body size by their teenage years, also rather like us.

So blue whales have a phenomenal overall growth rate: from single fertilised egg cell to ~150-tonne leviathan within a couple of decades. And that means that they must experience phenomenal rates of cell division. In general, the individual cells of a blue whale’s different tissues are not substantially larger than those of the same tissues in other mammals, so they don’t get large by having large cells, but by having lots of them (although, as an aside, some of the single nerve cells in a blue whale’s spine are incredibly long, and may stretch by 3 cm per day to grow without dividing).

Cell division involves a risk. Every time a cell copies its DNA during division, there’s a chance of errors creeping into the copy. If an error arises at just one of many key places in the genetic code, it can trigger the development of different forms of cancer (although there are also mechanisms that try to spot the errors and correct them). Every cell division is a roll of the dice – and blue whales therefore seem to roll those dice more times than any other mammal.

So whales embody an apparent paradox for biologists, as this blog post by Carl Zimmer summarises nicely. Whales do get cancer, but if blue whales were just like us, then all of them should have colorectal cancer by the age of 80 (and probably other cancers too), yet most of them do not. Instead, they are an example of “Peto’s paradox”: the observation that cancer rates don’t seem to correlate with larger average body sizes among mammalian species.

Carl Zimmer’s excellent post summarises ideas about how whales might resolve Peto’s paradox, and ends by mentioning not having “a single fully-sequenced genome of a whale or a dolphin for scientists to look at” back in 2011. Well, three years is a long time in the field of genomics, and at last we do! Yim et al. (Nature Genetics, 46: 88-92, 2014) have now sequenced the genome of a Minke whale, and compared it with sequences from a fin whale, bottlenose dolphin and a finless porpoise, along with cows and pigs.

What their comparative genomic analysis shows is that some gene families appear to have expanded in the cetaceans, while others have reduced, compared with pigs and cows. Gene families associated with body hair and sense of taste or smell appear to be reduced in whales. But whales have expanded families of genes involved in combating oxidative stress in cells. Yim et al. suggest that these expanded gene families may be adaptations for prolonged breath-hold diving, which results in hypoxia (low oxygen conditions) in tissues.

During hypoxia, cells accumulate “reactive oxygen species” – potentially damaging forms of oxygen, such as hydrogen peroxide – and Yim et al. show that whales have expanded gene families in particular to cope with reactive oxygen species. Reactive oxygen species are also involved in the development of cancer, and hypoxia tolerance and cancer resistance have been linked in blind mole-rats, which live in low-oxygen conditions in their burrows. So what particularly strikes me initially from Yim et al.‘s study is that whales appear to have expanded families of genes that cope with hypoxia and reactive oxygen species – and they also embody Peto’s paradox.

This then poses a couple of speculative questions: did whales evolve prolonged breath-hold diving ability first, and in doing so acquire expanded gene families to combat oxidative stress, which might then have enabled the evolution of large body size by reducing cancer risk from reactive oxygen species?

Or did the evolution of whales first involve a selection pressure for large body size, resulting in the evolution of expanded gene families to combat oxidative stress, which were then co-opted to enable prolonged breath-hold diving?

In other words, I wonder which came first in cetaceans: prolonged breath-hold diving, or large body size? Estimating the body size of extinct cetaceans from incomplete fossil skeletons is tricky (as is inferring their diving capability), and of course it is possible that the two arose contemporaneously under simultaneous selection. But an analysis by Pyenson & Sponberg (Journal of Mammalian Evolution, 18: 269-288, 2011) indicates that extremely large body size may be a relatively recent phenomenon in the evolutionary history of the baleen whales (which include the blue whale).

There’s a lot of blog-post-arm-waving here, and I’m very conscious of straying outside my own area, into the fields of cetacean evolution and cancer biology. But perhaps it’s interesting to consider whether a reduced cancer risk in whales, implied by their huge body size, might involve a spin-off from their adaptations for extreme breath-hold diving – or vice-versa.

Jon Copley, May 2014

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2. Frilled shark
Look at those teeth. And the killer look!

3. Giant squid
Just compare the size of the scientists and that of the squid. It’s called giant for a good reason. The fact that the pic is black and white makes it even more terrifying (like a Hitchcock movie)

4. Vipefish
Thought to put the angler fish, but looking at that picture I changed my mind.

5. Moray eel
You are thinking “An eel? Seriously?”. Well yes, this particular eel always gave me the creeps. Notice the long, cold stare!

So which are your scariest sea animals?
Source:
1. http://www.stunninginterestingfacts.com/2013/01/the-blobfish.html
2. http://en.wikipedia.org/wiki/File:Chlamydoselachus_anguineus_head.jpg
3. http://upload.wikimedia.org/wikipedia/commons/3/3c/Giant_squid_Ranheim.jpg
4. http://i.telegraph.co.uk/multimedia/archive/01679/deep-sea-viperfish_1679474i.jpg
5. http://www.messersmith.name/wordpress/tag/moray-eel/

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